Is that a question?
ANY attack and ANY defence has weak points - however it is whether they can be noticed in time to react. Aikido attack and defence are radically different. Attacks are not intended to blend - they are just solid, fast, sinlge attacks of a formulaic nature. If uke and nage are equal within the encounter (i.e. competitive situaitons), you are right - the situation looses its realness because you get into a never ending spiral of response directed towards their last response.
We often specify the type of response uke makes for a particular technique, to enable nage to understand why we would e.g. move under the arm or enter rather than turn.
Sincerety in training, realisation of what we are simulating and constant directed practise is the only necessity - sometimes we get caught up in believing that the attacker knows exactly what we know.